1. Field of the Invention
The present invention relates to manufacturing galvannealed steel sheet used as an automobile rust-preventive steel sheet, and a galvannealed steel sheet.
2. Description of the Related Art
Zinc-based hot-dip plating and electroplating have been developed and industrialized to produce automobile rust-preventive steel sheets having excellent sacrificial anticorrosion ability. Particularly, galvannealed steel sheets are popularly employed as automotive steel sheets because of low manufacturing cost and high corrosion resistance.
The galvannealed steel sheet is a surface treated steel sheet of low cost and high corrosion resistance. When used as an automobile rust-preventive steel sheet, however, a problem in workability in press forming has been pointed out as compared with electrogalvanized steel sheets, because of the fact that the plating layer itself is composed from a Znxe2x80x94Fe-based intermetallic compound produced through mutual diffusion of the substrate metal and pure zinc, and many studies have been made to improve press-formability of the galvannealed steel sheet.
Problems are encountered in actual press forming of the galvannealed steel sheet.
One is a phenomenon known as powdering in which the galvannealing layer is peeled off into powder during working. A xcex93-phase, if produced in a large quantity on the galvannealing/steel sheet interface, causes deterioration of powdering resistance and press-workability. A galvannealed steel sheet having excellent powdering resistance is therefore demanded.
Another property to be satisfied during press working is associated with the condition of the surface galvanizing layer such as friction with a die.
These properties largely depend upon the phase structure of the surface of galvannealing layer, and the presence of a soft and low-melting-point xcex6-phase as compared with a xcex41-phase causes serious deterioration of properties.
A galvannealed steel sheet having good press-workability is a steel sheet satisfying both powdering resistance and low coefficient of friction. For this purpose, a galvannealing phase mainly comprising a xcex41-phase achievable by inhibiting the xcex93-phase and the xcex6-phase would be an ideal galvannealing phase.
Conventionally available methods for manufacturing a galvannealed steel sheet having satisfactory powdering resistance and low coefficient of friction, in which the phase structure is properly controlled, include controlling the Al concentration in the galvanizing bath, and a method of controlling generation of excessive xcex93-phase and xcex6-phase by setting forth the degree of alloying of the galvannealing layer.
Regarding alloying conditions applied when manufacturing a galvannealed steel sheet, on the other hand, effectiveness of regulating the alloying temperature has been reported.
When trying to obtain a galvannealed steel sheet mainly comprising a xcex41-phase through the usual process, it is difficult to obtain a targeted galvannealing phase structure by only regulating simply an alloying temperature. It is necessary to satisfy other requirements for a strict control of the galvannealing phase structure.
Some techniques have been introduced to date in view of the heating rate upon alloying as a factor.
For example, Japanese Unexamined Patent Publication No. 4-48061 discloses a technique comprising the steps of conducting alloying at a heating rate of at least 30xc2x0 C./second to a temperature within a range of from 470 530xc2x0 C., and regulating the relationship between the coating weight and the iron content in the plating layer, thereby improving press-formability.
Japanese Unexamined Patent Publication No. 1-279738 discloses obtaining a plating having excellent powdering resistance and flaking resistance by limiting the Al concentration in the plating bath within a range of from 0.04 to 0.12 wt. %, reaching an alloying temperature of at least 470xc2x0 C. in two seconds after the completion of the coating weight control, and rapidly cooling the plated sheet to a temperature of 420xc2x0 C. or less in two seconds after completion of alloying.
Japanese Unexamined Patent Publication No. 7-34213 discloses a technique of improving interface adhesion by using an Al concentration in the bath within a range of from 0.105 to 0.3 wt. %, subjecting the sheet to hot-dip galvanizing, then heating the same at a rate of at least 20xc2x0 C./second, performing alloying at a temperature within a range of from 420 to 650xc2x0 C., and heating the sheet at a temperature of from 450 to 550xc2x0 C. for a period of at least three seconds.
In order to manufacture a galvannealed steel sheet having excellent press-workability, as described above, the phase structure of the galvannealing layer must mainly comprise a xcex41-phase. An object of the invention, as described later, is to inhibit generation of the xcex6-phase and the xcex93-phase.
In this respect, the conventional art disclosed in the aforementioned Japanese Unexamined Patent Publication No. 4-48061 of improving press formability by heating the sheet at a heating rate of at least 30xc2x0 C./second, and regulating the relationship between the coating weight and the iron content in the plating layer inhibits generation of the xcex6-phase and the xcex93-phase to some extent, but press formability cannot be improved to a sufficient level by this means alone. A galvannealed steel sheet cannot be manufactured containing reduced xcex6 and xcex93 phases unless a sufficient amount of Al is kept in the galvanizing layer.
While Japanese Unexamined 4-48061 sets forth the relationship between the coating weight (W g/m2) and the iron content in the galvannealing layer (CFe wt. %) by making 18xe2x88x92(W/10)xe2x89xa7CFexe2x89xa79, an increase in the coating weight in this case leads to a narrow range of iron content in the galvannealing layer to be controlled, resulting in a problem of difficult operation.
The above-mentioned Japanese Unexamined Patent Publications Nos. 1-279738 and 7-34213 set forth the Al concentration in the galvanizing bath in addition to the alloying conditions.
However, when trying to ideally control the phase structure of plating, as described later, simple regulation of constituent concentrations in the plating bath is not sufficient. The conventional techniques described do not achieve the target of inhibiting generation of the xcex93-phase and the xcex6-phase significantly.
The present invention provides a manufacturing method for a galvannealed steel sheet, comprising the steps of subjecting a steel sheet to hot-dip galvanizing, then heating the sheet at a heating rate of at least about 10xc2x0 C./second to a maximum sheet temperature within a range of from about 470 to 550xc2x0 C., subjecting the sheet to an alloying treatment at a temperature of up to the maximum sheet temperature, controlling the Al content expressed as XAl% of the galvannealing layer and the coating weight expressed as W g/m2 to satisfy substantially the following equation (1), and obtaining a Znxe2x80x94Fe galvannealing layer having an iron content of from about 7 to 12%; a galvannealed steel sheet having intensity of a prescribed interplanar spacing of xcex6-phase, xcex41-phase and xcex93-phase as determined through X-ray diffraction applied to the galvannealing layer by peeling off the galvannealing layer at the galvannealing/steel sheet interface, substantially satisfying the following equations (4) and (5); and a galvannealed steel sheet excellent press workability, having a whiteness and glossiness substantially within the prescribed ranges:
5xe2x89xa6Wxc3x97(XAlxe2x88x920.12)xe2x89xa615xe2x80x83xe2x80x83(1)
I(xcex6:1.26)/I(xcex41:2.13xe2x89xa60.02xe2x80x83xe2x80x83(4)
I(xcex93:2.59)/I(xcex41:2.13)xe2x89xa60.1xe2x80x83xe2x80x83(5)